1. Field of the Invention
The invention relates generally to global positioning system devices and navigation receivers and more specifically to methods and apparatus for providing continuing position solutions in adverse signal conditions.
2. Description of the Prior Art
Triangulation, range navigation, hyperbolic navigation, etc., have long been used to provide position solutions for navigation. The comparatively old OMEGA navigation system used for aircraft navigation typically required that the aircraft be in receipt of signals from at least three fixed ground stations, and was like the global positioning system (GPS) in that time was an unknown. A "half dead reckoning" method was developed that allowed partial, less precise navigation to continue when only two OMEGA ground stations were being received.
In the global positioning system (GPS), more than just the three-dimensional position must be determined. The time must also be known very precisely. Therefore, each GPS navigation receiver ordinarily requires that the signals from at least four orbiting satellites be received to compute the four dimensional solution that includes time. Since the orbiting satellites are ever changing in their three dimensional positions relative to the receiver, it is preferable that the constellation geometry not include any two satellites that have too acute an angle between them, e.g., for the best position dilution of precision (P-DOP).
Recent GPS technology developments have been made that deal with ground vehicle navigation and the urban canyon problem and the complete loss of signals from moving through tunnels and buildings. The urban canyon restricts the visible satellites to those with the highest elevation, e.g., near zenith. Some of the solutions proposed have included dead reckoning system integration with the GPS navigator. For example, turning rate sensors and speedometers mounted on cars and trucks have been used to propagate the position solutions conventionally provided by the GPS navigator during GPS signal blackout or periods of partial loss.
Where the altitude is known or can be assumed, the signals from a minimum of three orbiting GPS satellites can be used to provide navigation. For example, the altitude of a ship on the ocean can be assumed to have an altitude of sea level. This is similar to having available a fourth satellite at the center of the earth. The earth surface position and time of day can be used to look up the tide in a database to make the altitude assumption even more precise. Similarly, the altitude of an airplane can be obtained from an altimeter, e.g., corrected for local barometric pressure.
It can happen, for a variety of reasons, that only two GPS satellites are visible to the navigator. But it is not always the case that those two will have similar elevations and azimuths that prevent a favorable geometry for a solution that describes a line.